Field of the Invention
The present invention relates to an image pickup apparatus that performs focus detection.
Description of the Related Art
Conventionally, an image pickup apparatus has been known that covers a pair of object images (optical images) into electric signals (image signals) by using an AF sensor to detect a focus state of an image pickup optical system based on a phase difference of a pair of image signals. The focus state of the image pickup optical system is detected while a focus detection region is changed depending on an object, an imaging lens, or setting of a camera.
Japanese Patent Laid-open No. 2006-220684 discloses a configuration in which a focus detection region (signal accumulation region) is changed to increase the number of the focus detection regions such that a focus detection time can be reduced even when a defocus amount is large. Japanese Patent Laid-open No. 2002-162252 discloses a configuration in which an optimum focus detection region (signal accumulation region) is determined in accordance with setting of a range of driving a distance ring of an imaging lens so as to achieve high-speed processing and prevent erroneous focus detection without unnecessary calculation processing.
However, when the accumulation region (focus detection region) of the AF sensor is changed as disclosed in Japanese Patent Laid-open Nos. 2006-220684 and 2002-162252, a center position of the focus detection region may be changed. A typical image pickup apparatus calculates a defocus amount by using a correction value depending on each AF frame (focus detection point) for an image shift amount obtained by a correlation calculation in the focus detection. In this case, if the center position of the focus detection region is changed, a precise correction cannot be performed and it is difficult to calculate a highly-accurate defocus amount. Recently, an image pickup apparatus in which highly-dense AF frames over 40 points are settable has been proposed, and there is a remarkable tendency to change the center position of the focus detection region and accordingly to reduce the focus detection accuracy (calculation accuracy of the defocus amount).